fix(datasource): align access validation in legacy views

[sha174n]

SUMMARY

This PR updates the legacy datasource views to ensure consistent resource-level validation across metadata and save endpoints.

Changes:

• Validation Alignment: Integrated security_manager.raise_for_access into the get, external_metadata, and external_metadata_by_name methods to align with standard application patterns.
• Ownership Verification: Refactored the save method to ensure resource ownership is validated for all update requests, regardless of the specific structure of the request payload.
• Protocol Consistency: These updates ensure that the older /datasource endpoints follow the same validation protocols used in the modern REST API layers.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

(Note: Please re-attach the screenshots/GIFs that were previously in this section.)

TESTING INSTRUCTIONS

Automated Tests:
Verify the authorization logic by running the unit test suite:

pytest tests/unit_tests/views/datasource/views_test.py

Manual Verification:

1. Check Metadata Access: Attempt to call the legacy get or external_metadata endpoints using a datasource ID that the current user is not authorized to view.
2. Verify Response: Confirm the application correctly denies the request (returns a 403 or redirects) rather than returning the metadata.
3. Check Update Logic: Call the /datasource/save/ endpoint with an update to a datasource the user does not own, and verify the request is blocked even if the owners field is omitted from the request data.

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 64.24%. Comparing base (ed622e2) to head (54c90ee).
⚠️ Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
superset/views/datasource/views.py	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #38647      +/-   ##
==========================================
- Coverage   64.99%   64.24%   -0.75%     
==========================================
  Files        1819     2532     +713     
  Lines       72515   129547   +57032     
  Branches    23149    29905    +6756     
==========================================
+ Hits        47128    83231   +36103     
- Misses      25387    44864   +19477     
- Partials        0     1452    +1452     

Flag	Coverage Δ
hive	40.59% <0.00%> (?)
mysql	61.59% <80.00%> (?)
postgres	61.66% <80.00%> (?)
presto	40.61% <0.00%> (?)
python	63.27% <80.00%> (?)
sqlite	61.29% <80.00%> (?)
unit	100.00% <ø> (?)

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